You could sponsor him on patreon instead, it might even come with a few non-reddit perks: https://www.patreon.com/syncforreddit

Sync has always meant Sync for reddit so I would interpret the lifetime Sync Ultra subscription as the lifetime of the project which is indeed shutting down. The new app will probably have similar monetization and may grandfather in the subscriber of the old program but I would not consider it obvious. The fact that there are no direct donations options changes my perspective and I agree with you that in that case it would be a good way to show support. I think I will wait and buy the new app as soon as it is out, this way I will still contribute financially, I will boost sales and so visibility of the new project and I will not have to worry about having the perks from the old subscription hereditated to the new one. If Sync for Lemmy takes off it could be interesting to set up an official instance with associated something like liberapay for donations, but right now I wouldn't take the focus away from the Android client.